1. Field of the Invention
This invention relates to cooled gas turbine vanes and more particularly to hollow vanes housing an insert having apertures directing jets of cooling air against the internal walls of the vane.
2. Description of the Prior Art
Hollow, air-cooled gas turbine vanes containing an insert for directing the cooling air to impinge against the internal walls of the vane are known in the art as exemplified by U.S. Pat. Nos. 4,056,332 and 3,767,322, with the latter patent and the present invention having a common assignee. The cooling air, after impinging on the inner walls of the vane is normally exhausted into the gas turbine motive gas flow path. A portion of the air may be exhausted through side openings in the vane walls to provide a protective layer of air adjacent the exterior surface of the vane for film cooling and another portion may be exhausted through a trailing edge outlet in the form of a radially extending narrow passage or slit from the internal chamber which also cools the area of the vane adjacent the trailing edge.
It has been suggested in the prior art air cooled vanes to employ pin-like members extending transversely within the trailing edge outlet to generate turbulence in the flowing air to improve the convective heat transfer between the air and the adjacent vane wall of the slit. The pins also serve as mechanical support to maintain the exhaust passage or slit dimensionally stable and minimize thermal distortion that could cause unpredictable air flow therethrough.
It was found that the pin height has no appreciable effect upon the heat transferred to the air flowing thereacross, and the greatest cooling effect was provided by turbulent air flow adjacent the internal wall surfaces of the slit. However, because of the generally broad entrance or throat leading to the downstream narrowed exhaust slit, the cooling air velocity, at its entrance, was generally insufficient to generate turbulence therein as it flowed around the initial pins in this entrance area. Thus, to maximize the heat transfer to the cooling air in the vicinity of the entrance area it was necessary to increase the air velocity across the initial pins sufficiently to cause turbulence in the air flow adjacent the slit wall surfaces. (The air velocity to the downstream narrowed passage of the slit was sufficient to cause turbulence in this downstream area providing adequate heat transfer to the air flowing therethrough.)
Two obvious alternatives for increasing the entrance velocity of the cooling air are (1) increase the volume of air flowing therethrough or (2) decrease the entrance throat area to the slit. However, because the efficiency of the turbine is reduced with each incremental increase in cooling air flow, it is preferable to maintain the volumetric flow rate of such cooling air to the vanes at a minimum. Further, because of the variations in heat absorption rates and resulting rates and amount of thermal expansion induced thereby along the vane walls and the attendant stresses associated therewith, it is preferable not to decrease the throat or entrance area to the slit by any sudden or abrupt increase in the wall thickness of the vane.